1. Field of the Invention
The present invention pertains to the field of electronic pick-up devices for electric musical instruments. In particular, the present invention pertains to a pick-up device that reduces background hum noise while maintaining high-quality sound reproduction.
2. Description of the Related Art
The present invention relates to a pick-up device for an electric instrument having one or more strings, such as an electric guitar. When a person plays a stringed electric instrument, the strings vibrate with harmonic frequencies. A pickup assembly senses the vibration of the strings and ideally generates an electronic signal containing the same harmonic frequencies without any distortion. The electronic signal is communicated to an amplifier and speaker system to generate sound reflecting the vibration of the strings.
FIG. 1 is a schematic diagram of a first prior art pick-up device 100 having a magnetic coil 102, a first variable resistor 104 and a first audio jack 106. The magnetic coil 102 generates a magnetic field that encompasses the strings of the instrument. The vibration of the strings within the magnetic field causes current to flow through the magnetic coil 102 with a frequency characteristic representing the string vibrations, as is well known to one of skill in the art. Thus, the vibrations of the strings induce an electronic signal within the magnetic coil 102 that is communicated to a first audio signal line 108. The audio signal on the first audio signal line 108 is attenuated by the first variable resistor 104, which implements a volume control. The attenuated audio signal is communicated to the first audio jack 106, and through the first audio jack 106 to an amplifier circuit. The amplifier circuit amplifies the audio signal to a sufficient power level to drive one or more speakers. Thus, the vibrations of the strings of the instrument are converted into corresponding sound at the speaker.
The pick-up device 100 produces excellent sound quality. The harmonic frequencies of the vibrating string, that are within the audible range, are accurately reproduced as sound waves at the speaker. However, in many environments, the pick-up device 100 also produces a humming noise at the speaker. This humming noise is typically caused by the effect of electrical devices within the surrounding environment that operate off the main AC power line. These electrical devices generate electromagnetic fields that also affect the signal generated by the magnetic coil 102. Thus, the audio signal on the first audio signal line 108 has a music component caused by the vibration of the strings and a noise component caused by externally generated electromagnetic fields. Because the main AC power line is typically a 60 Hz signal, the noise component of the signal on the first audio signal line 108 contains a strong 60 Hz frequency component, although other frequencies may also be present.
FIG. 2 illustrates a second prior art pick-up device 150 designed to eliminate the humming noise caused by external electromagnetic fields. The pick-up device 150 has a first primary coil 152 and a first secondary coil 154, each of which generate both a music component and a noise component. The first coils 152, 154 have their magnetic fields reversed from one another, and they are wound in opposite directions. Winding the coils in opposite directions causes the noise components generated by the first coils 152, 154 to have opposite phase, so that the noise components substantially cancel each other. However, the reversed magnetic fields, in addition to the opposite winding directions, causes the music components generated by the first coils 152, 154 to have the same phase. Thus, the music components are added together, while the noise components substantially cancel each other.
Although the pick-up device 150 can be designed to substantially eliminate the background humming noise, the sound quality produced by the hum filtered pick-up device 150 is not as good as the sound quality of the nonfiltered pick-up device 100. The addition of the first secondary coil 154 adversely affects the frequency response of the pick-up device 150, primarily because of the impedance of the first secondary coil 154. The inductance and capacitance, in particular, of the first secondary coil 154 adversely affects the frequency response of the first primary coil 152. Similarly, the inductance and capacitance of the first primary coil 152 adversely affects the frequency response of the first secondary coil 154.
FIG. 3 illustrates a third prior art pick-up device 190 that is described in U.S. Pat. No. 4,581,974, issued to Fender on Apr. 15, 1986. Similar to the pick-up device 150, the pick-up device 190 provides a first coil 172 and a second coil 174 for hum cancellation. The pick-up device 190 also provides some isolation between the two coils 172, 174 to reduce the effect that the impedance of one coil has on the frequency response of the other coil. However, the tone quality produced by the pick-up device 190 is still significantly worse than the tone quality of the nonfiltered pick-up device 100. The frequency response of the two coils 172, 174 is still adversely affected by the impedances surrounding the two coils 172, 174. Also, the music component of the audio signal is subjected to the frequency response of the operational amplifier 170.